The purpose of cryptography is to exchange messages in perfect privacy between a transmitter and a receiver by using a secret random bit sequence known as a key. Once the key is established, subsequent messages can be transmitted safely over a conventional channel. For this reason, secure key distribution is a fundamental issue in cryptography. Unfortunately, the conventional cryptography provides no tools to guarantee the security of the key distribution because, in principle, classical signals can be monitored passively. The transmitter and receiver have no idea when the eavesdropping has taken place.
However, secure key distribution is possibly realized by using the technology of quantum key distribution (QKD). Quantum key distribution is believed to be a natural candidate to substitute conventional key distribution because it can provide ultimate security by the uncertainty principle of quantum mechanics, namely, any eavesdropping activities made by an eavesdropper will inevitably modify the quantum state of this system. Therefore, although an eavesdropper can get information out of a quantum channel by a measurement, the transmitter and the receiver will detect the eavesdropping and hence can change the key.
A variety of systems for carrying out QKD over an optical fiber system have been developed. Quantum cryptography has already been applied to the point-to-point distribution of quantum keys between two users. As shown in FIG. 1, quantum cryptography system in the prior art employs two distinct links. Of them, one is used for transmission of a quantum key by an optical fiber, while the other carries all data by internet or another optical fiber.
However, it is desirable to apply quantum cryptography in currently deployed commercial optical network. Yet only several studies on quantum key distribution over 1,300 nm network have been reported to date. One problem of the reported system is that it is difficult to transmit signals over a long distance at 1,300 nm in standard single mode fibers. Thus, quantum key distribution with wavelengths around 1,550 nm over the long distance is preferred. In addition, it is considered that no strong signals (e.g. conventional data) should exist in network with quantum channels or that a large spacing of wavelengths between a quantum channel and a conventional channel is needed to lower the interference from the strong signal.
However, this is not true in the installed commercial optical network because there are many strong signals that can cause severe interference to the quantum channel in the current optical fiber communications network employing WDM transmission.